Concrete is an essential material for building all types of infrastructure including buildings, roads, dams, etc. As for brittle materials in general, concrete is strong under compression and weak under tension or flexure and has the tendency to crack and has poor thermal conductivity. The exothermic reaction in the formation of concrete and the poor thermal conductivity of the materials in concrete pose a problem for large pours such as dams, where coolant systems must be built into the slabs for proper curing.
The use of steel or fiber (glass or carbon) reinforcements in concrete has major drawbacks. For example, steel fibers can only be used on surfaces that can tolerate or avoid corrosion and rust stains. In environments where the structure is exposed to rapid heating and cooling, the thermal mismatch between concrete and steel may introduce extensive spalling in concrete specimens because of pore pressure build up in regions of severe temperature gradients. Glass fiber is inexpensive and corrosion-proof, but not as strong as steel and will not increase the concrete tensile strength. Using carbon fibers in concrete adds complexity to the manufacturing process.
Approaches to monitoring the structural health of concrete using external sensors suffer due to many different technical complexities. External piezo-electric sensors and embedded sensors approaches require foreign materials to be either embedded or attached to the structure. This adds complexities in processing and is not cost-efficient for large applications. Also, the performance of these materials depends on the conditions of the surrounding environment. Short carbon fiber is not practical for large applications. Also, the electrical performance of the carbon fibers is not significant enough to produce feedback signals from the structure.
Carbon nanotubes are quasi-one dimensional, nearly single crystalline (axially), hollow, graphitic carbon structures. The combination of high aspect ratio, small size, excellent mechanical properties, low density, and high electrical conductivity make them perfect candidates as fillers in polymer composites. Experimental as well as theoretical predictions on nanotubes suggest an axial Young's modulus of 1 TPa.
These exciting properties make carbon nanotubes a greatly desired carbonaceous material that has wide range of applications for its extraordinary physical, chemical, and mechanical properties. However, carbon nanotubes' lack of proper dispersion and tendency to aggregate in aqueous environments prevent them from being used in many applications. As prepared nanotubes are insoluble in many liquids such as water, polymers etc. A good dispersion of the materials, preferably up to single nanotube level, is of critical importance in achieving the predicted exciting properties for nanotube reinforced materials.
Needs exist for improved methods of reinforcing cementitious materials. Also, the recent catastrophic failures of civil structures, such as bridges, dams, levees, buildings, etc., echo the need for an in-situ monitoring of the health and integrity of structures.